Dual mode filters are widely used in different kinds of microwave systems, for instance in such systems as high data-rate communication systems, high resolution radars, etc. In such systems, an important parameter is compact size.
In a dual-mode filter, two orthogonal modes occur at a resonator frequency. A conventional dual mode filter comprises a substrate of a non-conducting material, on which two conductors (“lines”) extend between an input and an output port of the filter, with one of the lines being longer than the other. One of the two conductors is made to comprise a so called “perturbation element” in the middle of the conductor's extension between the input and the output port, in order to get the dual mode effect, which is also obtained due to the difference in lengths of the two conductors.
Often, the two conductors are designed to meander on the substrate, in order to shrink the total area of the substrate. However, a drawback of a meandering design is insertion losses and the surface area necessary for the dual mode filter.